This invention relates to powder spray coating and, in particular, to the recovery of powder which has not adhered to the article to be coated from a powder spray booth.
The powder which does not adhere to an article to be coated in a powder spray booth is known as the xe2x80x9coversprayxe2x80x9d. Overspray powder may miss the article to be coated, rebound from a surface of the article, or be deflected by the electrostatic fields in the spray booth away from the article.
It is known to recover overspray powder by exhausting the air from the booth and hence the air-borne overspray powder. The filter elements which have been used to recover overspray from powder spray booth are capable of removing substantially all the overspray from the spray booth exhaust air so that the powder may be reused. It is also known to use cyclone separators instead of, or in conjunction with filter elements for recovery of overspray. Cyclone separators are effective in removing the majority of the overspray from the spray booth exhaust air and are easier to clean than filters because very little powder is deposited inside the cyclone housing.
In known arrangements the cyclone separators have been connected to the powder spray booth by a duct extending from an overspray intake near the floor of the booth to the inlet of the or each cyclone. European Patent Application No. 94929600.8 discloses a powder coating system where the duct is built into the powder spray booth, with one of the side or end walls of the booth forming part of the duct.
In European Patent Application 98940476.9 there is disclosed a further development wherein the portion of the side or end wall which forms part of the duct is removable from the remainder of the wall to allow easy cleaning of the duct. That Application also discloses a system for collecting deposited overspray powder comprising a scraper bar which continuously reciprocates across the booth floor between the end walls to collect powder on the booth floor and move the collected powder towards the end walls of the spray booth adjacent to each of which overspray intakes are provided.
In recent years there has been a growing desire to be able to spray multiple colours in a single booth in rapid succession. However, changing powder colour requires cleaning of the booth to remove powder deposited on the walls, ceiling and floor which was not extracted by air exhaustion. Proper cleaning of the walls and floor of the powder spray booth between colour changes is very important to ensure acceptable production quality. Such cleaning may be carried out manually by an operator using a scraping and/or compressed air jet apparatus. However booths have been proposed which are intended to automate the deposited overspray powder cleaning process including the booth of European Patent Application No. 98940476.9 referred to above with its reciprocating scraper bar.
Whilst many developments have been made, there is a customer driven demand for even more rapid cleaning capability to reduce cleaning time still further. Furthermore, whilst automatic mechanical devices such as the reciprocating scraper bar of European Patent Application 98940476.9 do accelerate the cleaning process, they also increase construction cost and can be vulnerable to mechanical failure.
Cleaning is preferably conducted from one end of the booth to another and so, from the point of view of cleaning, the overspray intake of the powder recovery system would ideally be located at one end of the booth. However, from the point of view of spraying, the best location for the overspray intake is commonly held to be in the lengthwise centre of the booth to give balanced extraction air flow along the booth and therefore even air flow across the openings which are provided for the spray devices. When the overspray intake is provided at one end of the booth, attempts have therefore been made to modify the booth cross-section to produce the desirable air flow balance. In one known system the booth floor has side portions which slope downwards and the region therebetween is vertically offset to define a trough between the sloped side portions. A triangular insert is positioned in the trough during spraying. The combination of the sloped side portions and triangular insert helps balance the air flows along the length of the booth. However difficulties arise when the powder colour is to be changed as the triangular insert has to be removed for cleaning which adds to the cleaning time.
In another arrangement, shown, for example, in U.S. Pat. No. 4,715,314, the booth floor is formed with a slot which extends the length thereof and connects to a duct also running the length of the booth. However the slot can accumulate powder when one colour is sprayed for a long period of time which makes cleaning more difficult.
It is also been proposed to reduce cleaning time by preventing powder from ever depositing on the floor of a spray booth. German Patent 3408014 describes a system where the floor of the booth is formed as a series of descending steps and air is blown across each step from the slot between it and the step thereabove. The aim is to provide an air stream across the width of the booth which will carry all powder falling towards the floor into the overspray intake and prevent the powder reaching the floor and depositing there. The step form of the booth floor however increases the overall complexity of the booth and this, and the fact that air has to be continuously supplied, makes the booth expensive.
German Patent Application No. 19644360 describes a booth which is also intended to prevent overspray powder reaching the floor. The booth has one or more nozzles for creating a generally horizontal airflow across the booth floor. The air flow diverts descending overspray powder towards the inlet of a powder recovery system and prevents the overspray powder from depositing on the floor. In one embodiment, a central baffle positioned above the floor creates an open-sided duct leading to the inlet and a nozzle directs air to each side. As with the system of German Patent 3468014 a continuous air supply is required.
As noted above, booths in which part of the floor is sloped are known. However, according to U.S. Pat. No. 3,905,785, sloping alone is insufficient to cause all the overspray powder to flow by gravity to the recovery system. The Patent proposes a booth in which the floor is constituted by at least one air-permeable inclined plate. Air is sucked from the booth through the plate which acts to filter out air borne overspray. Periodically air is blown up through the plate to fluidise the deposited overspray thereon and cause it to flow to a recovery system. The suction period may be five to twenty times longer than the blowing period. The concept is similar to that of cartridge filters which are periodically purged by reverse air jets.
The majority of known booths are rectangular. However, it is also known to provide circular booths. A circular booth is disclosed in, for example, EPA 0839522. A problem with known circular booths is that these have conical floors leading to a suction duct by which overspray powder is transferred to a recovery system. A pit is generally required to accommodate the cone and duct in order that articles to be sprayed can be received at the same level as other elements in the production line.
It is an object of the present invention to provide a colour spray apparatus in which powder colour can be changed quickly and easily and which, moreover, is of relatively simple construction.
It is another object of the present invention to provide a powder coating system in which both air borne and deposited overspray can be recovered by a system involving a small number of relatively simple parts which are themselves readily cleaned.
It is a further object of the present invention to provide a powder spray booth in which the collection of deposited overspray is facilitated whilst still balancing the extraction air flow along the length of the booth.
It is a still further object of this invention to provide a powder spray booth of generally circular configuration which can be easily incorporated in a production line and which is simple to clean.
The invention will now be summarised with respect to four principal aspects: a diverter plate; sloped floor with cleaning air; pulsed cleaning air apparatus, and pulsed cleaning air method.
In one aspect the present invention provides a powder coating system comprising a powder spray booth with walls and a floor having two side portions and a downwardly offset region therebetween which defines a trough between the side portions, one or more powder spray devices positioned with respect to the booth walls to spray articles passed through the booth via openings in the walls, a powder recovery system having an overspray intake communicating with the interior of the booth, the powder recovery system including suction means for drawing overspray powder from the booth interior via the overspray intake, and, a diverter plate held within the booth between the floor side portions such as to close at least part of the trough and form a floor duct which is in communication with the overspray intake, the diverter plate including at least one aperture for the passage of overspray powder deposited thereon to the duct.
It has been found that with this apparatus it is possible to even out air extraction to give balanced air flow in the booth whilst still providing an arrangement which is easy to clean particularly if, as is preferred, the diverter plate is movable to a second raised position allowing access to the trough. The provision of at least one aperture allows overspray powder deposited on the diverter plate to be drawn therethrough and into the powder recovery system.
Preferably the side portions are sloped down towards the diverter plate. The diverter plate may define a slot between it and each of the side portions.
Deposited overspray powder on the side portions will trickle down towards the diverter plate and be extracted by the air flow to the powder recovery system via the slots between the plate and the side portions. As already noted, deposited overspray powder on the surface of the plate will be extracted via the apertures therein. As a consequence the amount of deposited overspray powder in the booth at the start of the cleaning operation will be reduced so reducing the scale of the cleaning operation and consequently its duration.
The booth may have any desired configuration. It may have the common rectangular shape with the longitudinal axis running generally horizontally. Alternatively, the booth can be circular with the central axis running generally vertically. The provision of an air distribution duct in a circular booth formed by the diverter plate enables limitation of powder build up and optimisation of air distribution and velocities in the booth through control of the size and shape of the apertures in the diverter plate. Advantage can therefore be taken of the easy cleanability of circular booths without requiring that a special pit be provided to position the booth in a production line. This is a significant benefit as is the fact that operators can safely walk into the booth which is not possible with known circular booths having a conical base.
In a rectangular booth, the overspray intake may be provided at or near one end of the booth with the diverter plate extending from that end along part of the length of the booth. In a rectangular booth provided with a plurality of automatic powder spray devices and having slots in the side walls for the automatic powder spray devices, the length of the diverter plate may be the distance between the end and the gun slot furthest from the end plus an offset. The offset can be comparatively small, a suitable value for a common booth design being 200 mm.
It has been found that it is only necessary to provide the diverter plate in the part of the booth where automatic guns are employed. This is because there are a higher number of guns in the automatic gun portion of the booth and therefore more powder flow into the booth there from the guns. Also, this part of the booth is closest to the extraction fan. Without the diverter plate, the fan can distort the powder spray patterns from the guns and may draw the powder into the collection system before it has a chance to adhere to the part. The effect of the diverter plate is to equalize the suction force of the fan so that powder has more time to adhere to the part and oversprayed powder is drawn evenly towards the fan. In the manual gun section of the booth, there are fewer guns and they are farther away from the fan, so the patterns from those guns are not affected to such a great extent by the fan. Therefore, the diverter plate is not as necessary in the manual gun section of the booth.
In a rectangular booth of the design shown in European Patent Application No. 98940476.9 with an overspray intake at each end, a diverter plate may be provided extending from each end. It has been found that a booth with air intakes at both ends in fact produces the best balancing of the air flow in the booth and actually results in a xe2x80x9cdeadxe2x80x9d zone in the middle of the booth where powder coating is particularly efficient since powder particles have an opportunity to adhere to the part relatively unaffected by the collection system air flows. The plates can be arranged to extend to either side of this xe2x80x9cdeadxe2x80x9d zone.
In a particularly preferred embodiment, the diverter plate is formed in two or more sections, the diverter plate sections being arranged at different vertical spacings from the bottom of the trough such that the duct has different heights along its length. It has be found with this arrangement it is possible to improve still further the air flow balance in the booth and to obtain more even suction.
In this embodiment, the plate section or sections adjacent the or each overspray intake may have the greatest vertical spacing from the trough bottom. With a rectangular booth having an overspray intake at one end, the sections may be arranged in a descending series of steps towards the other end. With a circular booth having a centrally located overspray intake, the section thereabove may be at maximum spacing with one or more lower sections on either side. The narrowing of the duct away from the overspray intake produces the desired even suction and more balanced air flow.
The diverter plate is suitably provided with a plurality of apertures arranged in a pattern. With a rectangular booth, the pattern may be triangular with the apex being at the end of the booth closest to the fan. This pattern has been found to give the best combination of powder collection and air balancing. The total hole cross-sectional area gradually increases from the end towards the middle of the booth.
The apertures in a preferred form are elongate in the lengthwise direction of the plate to give maximum circumference for given cross sectional area and therefore maximum ability to draw deposited overspray from the upper surface of the diverter plate.
In accordance with a second aspect, the present invention provides a powder coating system comprising a powder spray booth having floor and walls, one or more powder spray devices positioned with respect to the walls to spray articles passed through the booth via openings in the walls, and a powder recovery system having an overspray intake communicating with the interior of the booth, the powder recovery system including suction means for drawing overspray powder from the booth interior via the overspray intake, wherein at least a portion of the booth floor is sloped and wherein cleaning air supply means is provided for directing air down the sloped portion(s).
Whilst it has been known to make at least a portion of booth floors sloping, the intended result of this, that deposited overspray powder will eventually trickle down the sloped portion, does not always occur. There can be significant build ups of deposited overspray powder on the sloped portion or portions which adds to cleaning time and also gives rise to danger of explosions. By providing a cleaning air supply which directs air down the sloped portion or portions, downward movement of deposited overspray powder is ensured to bring the deposited overspray powder to a region where it can be easily cleaned, either manually or automatically by a mechanical scraper or the like and/or extraction in the exhaust air of the powder recovery system.
The cleaning air supply means may comprise a source of air and at least one plenum extending along at least part of the sloped portion and located at the upper edge of the sloped portion, the plenum having at least one air outlet, preferably plural air outlets.
Very preferably the air source provides periodically pulses of air to the air outlets. It has been found that it is an unnecessary waste of energy to constantly direct air across the slope portion or portions and it suffices just to pulse the air to start movement of deposited overspray powder down the sloped portion or portions. It is currently believed that the best effects are achieved when the cleaning air supply means directs the air generally parallel to the surface of the sloped portion or portions, preferably at an angle of 1-3xc2x0 to the sloped portion or portions.
In the particularly preferred configuration of the first aspect, the cleaning air supply means of the second aspect may direct air down the side portions along the length of the diverter panel.
In accordance with a third aspect, the present invention provides a powder spray system having a floor, and walls, one or more powder spray devices positioned with respect to the walls to spray articles passed through the booth via openings in the walls, and a powder recovery system having an overspray intake communicating with the interior of the booth, the powder recovery system including suction means for drawing overspray powder from the booth interior via the overspray intake, wherein a cleaning air supply means is provided comprising a source of air and at least one plenum with plural air outlets, the air source providing pulses of air to the air outlets and the plenum being arranged to direct the air pulses across an internal surface of the booth on which overspray powder is deposited.
It has been found that pulses of air may be effective for collection of deposited powder across any internal surface of the booth not just sloped surfaces. The pulses of air cause the deposited overspray powder to collect, either by falling in the case of the ceiling or walls, or by movement along or across the booth in the case of the floor and so facilitate extraction of the deposited overspray powder by the powder recovery system. Good results are achieved without the need for a constant air flow or a complicated structure for directing the air. Thus the system is very economical both as regards manufacture and operation.
The air source may be a compressed air source, one or more fans or an air supply of the powder recovery system. The former has the advantage of cheapness whilst the latter makes for compactness and quietness since fans are relatively noisy.
Very preferably the first aspect is combined with the second and/or third which results overall in a very significant reduction in cleaning time coupled with even air distribution across the booth which produces higher transfer efficiency since the powder particles are given a greater opportunity to adhere to the part. These advantages are accomplished while good powder containment within the booth is also being achieved.
The invention also provides in a fourth aspect a method of operating a powder coating system including a powder spray booth having a floor and walls, one or more powder spray devices and a powder recovery system with an overspray inlet communicating with the interior of the booth, the method comprising periodically directing jets of air across an internal surface of the booth to collect deposited overspray powder thereon.
The method thus involves providing air assist in the booth which facilitates collection and removal of deposited overspray powder.
The internal surface may be the floor and/or one or more walls and/or the ceiling of the booth.
The jets of air are preferably directed sideways across the surface, or lengthwise along the booth. In this way the powder is gathered together and easily collected by a cleaning operation along the booth which may be either manual or automatic.
It has been found that good results are achieved when the air jets are pulsed on for five seconds for every five minutes of spraying of articles in the booth. This prevents any significant build-up of deposited overspray powder by causing removal of the majority of deposited overspray from the booth with the result that the inventory of powder moving around in the system, particularly the quantity within the booth itself, is kept to a minimum.
Alternatively or additionally the jets of air can be pulsed just prior to a colour change so that the amount of deposited powder which has to be removed during the cleaning operation is minimised.
Very preferably this fourth aspect is combined with the second aspect and the air jets are directed across at least part of a sloping portion of the booth floor. For effective movement of deposited overspray powder on the sloping portion(s), the air jets are directed generally parallel the sloping portion(s), preferably at an angle of between 1-3xc2x0 thereto.
Still a further aspect of the invention relates to the use of pressurized air to blow off overspray powder from the surface of the diverter plate. In one embodiment, the diverter plate has sloping surfaces, and an air plenum is provided along the apex to direct pressurized across the sloping surfaces. Overspray powder is thus blown off the diverter plate and into the floor duct below. The pressurized air may be a continuous flow or pulsed. The use of pressurized air to blow off the diverter plate may be used alone or in combination with the blow off feature of the sloped floor.
Still another aspect of the invention relates to the use of a vertically extending duct having a lower end in communication with the overspray intake and the floor duct. In one embodiment, the vertically extending duct has a removable portion such as doors that form part of the spray booth wall structure. The vertical duct removable portion is provided with one or more holes, slots or other openings to aspirate air from the spray booth during a spraying operation. Preferably the openings are located at an upper region of the vertical duct. Sliding wall sections of the spray booth may also be provided as required.
The invention also contemplates methods for cleaning a spray booth and methods for recovering overspray powder during a powder spraying operation using one or more of the above aspects of the present invention, as set forth in greater detail hereinafter.